The advancement in information technology related to optical fiber communications including the development of the internet and other information-sharing platforms can be undoubtedly regarded as one of the most transformative inventions in this era considering its magnificent contributions across all the fields. However, with the rapid increase in the data content and the number of internet users, network traffic has increased drastically. Since this trend is expected to continue, the development of alternative methods for overcoming the network capacity limit is highly desirable for future network capacity demands.
Previously, space-division multiplexing has been used to overcome the capacity limit of the current optical transmission systems. Interestingly, they can be used in combination with the current multiplexing techniques and can enhance the transmission capacity based on the propagation modes in the fiber. Consequently, the mode of a single-mode fiber can be converted to a propagation mode of few-mode fibers to realize mode-division multiplexing, whose performance depends on the accuracy and efficiency. Despite the availability of several methods for mode conversion, they have not achieved the desired accuracy and efficiency due to numerous limitations. As such, the development of alternative conversion methods is highly desirable.
Recently, Hokkaido University-based researchers: Tomohiro Maeda (PhD candidate), Professor Atsushi Okamoto, Professor Kazuhisa Ogawa, and Professor Akihisa Tomita developed a new wavefront superposition method for efficient and accurate mode conversion in mode-division multiplexing transmission. This method converted the input beam into the wavefront state comprising of conversion target and radiation modes of the few-mode fiber. Their work is currently published in the research journal, Applied Optics.
Unlike in conventional methods, where the output beam consists of only one conversion target, the output in this method could accommodate higher proportions of the target component. This was attributed to the ability to appropriately determine the weighting for the modes for more efficient conversion. Furthermore, no modal crosstalk was observed in the wavefront superposition method because all the radiation mode components in the output were eliminated by the mode-filtering property of the few-mode fibers.
To validate the feasibility of the proposed method, numerical simulations were used to compare its performance to that of the conventional method under optimal conditions taking into account the effect of the input beam diameter. Specifically, the applicability of the method was evaluated by performing mode conversions for few-mode fibers in large-scale mode-division multiplexing transmission systems supporting 3, 6, 10 and 15 propagation modes. The proposed method recorded the most accurate and relatively efficient mode conversion. For instance, higher efficiency of 2.4dB was reported even at an extremely low modal crosswalk. This higher performance is assumed to be due to the intensity distribution after modulation. Also, it required less attenuation even for the larger intensity of the input beam.
It was noteworthy that the output distribution was derived starting from the output of the phase modulation. Based on the findings, the wavefront superposition method is a promising solution for accurate and efficient mode conversation. In a statement to, Advances in Engineering, the authors hope their study will pave the way for exploring better distribution using optimization methods.
Maeda, T., Okamoto, A., Ogawa, K., & Tomita, A. (2019). Wavefront superposition method for accurate and efficient mode conversion. Applied Optics, 58(25), 6899.